AVCaptureDevice.setExposureModeCustom takes too long ?

Hello,

I am working on a fairly complex iPhone app that controls the front built-in wide angle camera. I need to take and display a sequence of photos that cover the whole range of focus value available.

Here is how I do it :

• call setExposureModeCustom to set the first lens position
• wait for the completionHandler to be called back
• capture a photo
• do it again for the next lens position.
• etc.

This works fine, but it takes longer than I expected for the completionHandler to be called back.

From what I've seen, the delay scales with the exposure duration. When I set the exposure duration to the max value:

• on the iPhone 14 Pro, it takes about 3 seconds (3 times the max exposure)
• on the iPhone 8 1.3s (4 times the max exposure).

I was expecting a delay of two times the exposure duration: take a photo, throw one away while changing lens position, take the next photo, etc. but this takes more than that.

I also tried the same thing with changing the ISO instead of the focus position and I get the same kind of delays. Also, I do not think the problem is linked to the way I process the images because I get the same delay even if I do nothing with the output.

Is there something I could do to make things go faster for this use-case ?

Any input would be appreciated,

Thanks

I created a minimal testing app to reproduce the issue :

import Foundation
import AVFoundation


class Main:NSObject, AVCaptureVideoDataOutputSampleBufferDelegate {
	let dispatchQueue = DispatchQueue(label:"VideoQueue", qos: .userInitiated)

	let session:AVCaptureSession
	let videoDevice:AVCaptureDevice
	var focus:Float = 0
	
	override init(){
		session = AVCaptureSession()
		session.beginConfiguration()
        session.sessionPreset = .photo
        
        videoDevice = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back)!
		super.init()
		
		let videoDeviceInput = try! AVCaptureDeviceInput(device: videoDevice)
        session.addInput(videoDeviceInput)
        
		let videoDataOutput = AVCaptureVideoDataOutput()
		if session.canAddOutput(videoDataOutput) {
			session.addOutput(videoDataOutput)
			videoDataOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA ]
			videoDataOutput.setSampleBufferDelegate(self, queue: dispatchQueue)
		}
		
		session.commitConfiguration()
		
		dispatchQueue.async {
			self.startSession()
			
		}
	}
	
	func startSession(){
		session.startRunning()
		
		//lock max exposure duration
		try! videoDevice.lockForConfiguration()
		let exposure = videoDevice.activeFormat.maxExposureDuration.seconds * 0.5
		print("set max exposure", exposure)
		videoDevice.setExposureModeCustom(duration: CMTime(seconds: exposure, preferredTimescale: 1000), iso: videoDevice.activeFormat.minISO){ time in
			print("did set max exposure")
			self.changeFocus()
		}
		videoDevice.unlockForConfiguration()
	}
	
	func changeFocus(){
		let date = Date.now
		print("set focus", focus)
		try! videoDevice.lockForConfiguration()
		
		videoDevice.setFocusModeLocked(lensPosition: focus){ time in
			let dt = abs(date.timeIntervalSinceNow)
			print("did set focus - took:", dt, "frames:", dt/self.videoDevice.exposureDuration.seconds)
			self.next()
		}
		videoDevice.unlockForConfiguration()
	}
	
	func next(){
		focus += 0.02
		if focus > 1 {
			print("done")
			return
		}
		changeFocus()
	}
	
	func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection){
		print("did receive video frame")
	}
}